1. Field of the Invention
The present invention relates to methods for correcting faults that appear in the manufacture of surface wave devices, especially reflective array compressors (RACs) that make use of the reflection of sound waves on variable-pitch arrays, inclined with respect to the direction in which these sound waves are propagated.
In these reflective array compressors, the product of the pass-band and signal time (called BT) of these filters is very great, which gives them their value.
2. Description of the Prior Art
An RAC, corresponding to the state of the art, is shown in FIG. 1. It has a piezoelectrical substrate 1, an interdigitated emission transducer 10 which, from the signal to be compressed, sends a sound wave towards a first reflecting array 100. This array has a series of gaps, in the form of lines that are inclined by substantially 45.degree. with respect to the propagation axis Z of the sound wave. These gaps, as well as the transducer 10 and the transducer 11 which shall be referred to further below, are formed by etching or metallization on the surface of the substrate.
The sound wave is reflected by the array 100, at substantially 90.degree., towards a second array 110 which is identical to the first array but is symmetrical with the axis Z. The sound ave is then reflected at substantially 90.degree. on this second array, and then sets off again in the direction opposite to the axis Z.
It is then collected by an output transducer 11 which it excites to give an output electrical signal.
The spacing between the gaps in the two reflecting arrays varies according to the distance from the input and output transducers. Thus, the reflective capacity varies with the frequency and, depending on this frequency, the sound wave will travel on a path such as 12. The length of this path and, hence, the time taken to travel on it, will depend on the frequency.
The total transfer function of this device corresponds to the product of the transfer functions of the input and output transducers and the transfer function of the reflective arrays, and gives rise to a compression of the pulse applied to the input transducer.
A device of this type does not directly give the exact amplitude and phase response sought in the useful band. This is because of various unwanted phenomena including, in particular, manufacturing defects and deformations of the piezoelectric substrate when it is being packaged.
In a known corrective method, though only for phase error, a conductive film of variable thickness and width is deposited between the two reflective arrays 100 and 110 to modify the propagation speed for each path of the waves and, therefore, for each frequency. However, this method cannot be used to correct errors that vary quickly with the frequency.
Furthermore, there are still the errors caused by the deformation of the piezoelectric substrate when it is being packaged.